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Priorities USA has now raised more than $155 million in the 2016 cycle

(CNN)Hillary Clinton's top-dollar super PAC enjoyed its best month of the campaign in September, a spokesman said Thursday.

And billionaire environmentalist Tom Steyer dedicated another $18 million to his climate group NextGen Climate, bringing his total contributions to the group to $56 million.

In total, Priorities USA raised more than $24 million, Justin Barasky said, meaning the group started October with $22 million in the bank.

"As the campaign enters its final weeks, Priorities is committed to ensuring Hillary Clinton wins by as much as possible, in as many states as possible, and is surrounded by Democrats in Washington who will join her in moving our country forward," Guy Cecil, the group's senior strategist, said in a statement.

With its September haul, Priorities USA has now raised more than $155 million in the 2016 cycle. Barasky said the group has another $17 million in commitments that will be donated before November 8.

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Because of an electoral map that is shifting in Clinton's direction, Priorities has decided to begin airing ads in Senate contests where Democrats are challenging incumbent Republicans.

To date, the group has begun running ads against Sen. Kelly Ayotte in New Hampshire and Sen. Pat Toomey in Pennsylvania.

These Senate moves seemingly come in place of helping Clinton in states like Utah, Arizona and Georgia, places that have become battlegrounds of late because of a slide by Republican nominee Donald Trump.